The present invention relates to a system and method for securing an integral closure bucket to a rotor wheel assembly. The integral closure bucket is inserted in a direction coincident with a radius of the rotor wheel assembly, where the remaining buckets have been previously inserted in a direction parallel to the axis of the turbine rotor wheel. More particularly, the present invention relates to a system and method for securing a closure bucket, having an integral shroud or closure, into a turbine rotor wheel assembly in which the remaining buckets can be axially inserted.
The buckets adjacent the closure bucket may be spread apart both tangentially and axially to provide sufficient clearance for the radial insertion of the integral closure bucket. Since the closure bucket is inserted radially, both the closure bucket and the adjacent buckets cannot be secured together with keys in the normal fashion, to prevent axial movement of the buckets. As a result, twist locks can be utilized to keep the buckets adjacent to the closure bucket from moving axially after assembly.
In the assembly of a turbine rotor wheel, the rotor blades, also known as buckets can be attached to the turbine rotor wheel by moving the buckets axially so that male dovetails on the buckets slide into mating female dovetails on the turbine rotor wheel. Both the male and female dovetails may be oriented to be parallel to the axis of the turbine rotor wheel.
Turbines wheels with buckets having integral covers or shrouds formed on their distal ends are known, as shown in U.S. Pat. No. 6,030,178, to Caruso, and assigned to Assignee of the present application, which is incorporated herein by reference. The shrouds have an approximately Z-shaped configuration so that the shroud of adjacent buckets nest with one another. As a result, when all the buckets are assembled on the turbine rotor wheel, the shrouds nest tightly with the adjacent shrouds creating a continuous circumferential coupling.
In operation, the buckets experience vibrational stresses that can reduce the turbine""s efficiency and lower the life expectancy of the bucket. One method for reducing the vibrational stress is to interlock the ends of the buckets with shrouds. In order to be effective, the shrouds should maintain sufficient contact between adjacent shrouds all the way around the circumference of the turbine rotor wheel. During operation of the turbine, centrifugal forces cause the radial growth and twisting of the buckets. To counteract these effects, the buckets and their covers can be assembled with compressive contact force between adjacent buckets.
Construction of turbine rotor wheels having radial-entry buckets is accomplished by assembling the buckets serially around the turbine rotor wheel so that the approximately Z-shaped shrouds nest with one another. However, when all the buckets, except the bucket, are assembled the shrouds on the first and next to buckets prevent axial insertion of the closure bucket. Consequently, the closure bucket must be inserted radially.
The patent to Caruso et al. teaches securing the closure bucket by employing an axial entry dovetail segment having generally radially opposite male dovetails extending generally in the axial direction. The turbine rotor wheel has conventional female dovetails at spaced positions about its circumference for receiving the male dovetails on the buckets or the axial entry dovetail segment. Instead of having a male dovetail, the closure bucket has a female dovetail to accept the male dovetail of the axial entry dovetail segment.
The foregoing construction has drawbacks. In particular, the z-shaped shrouds are designed to fit together very tightly. As a result, the clearance between the shrouds on the buckets adjacent the closure bucket location is insufficient to permit radial insertion of the closure bucket. The clearance is insufficient in both the axial and tangential directions.
Furthermore, in the turbine rotor wheel configuration taught in Caruso et al., the buckets are prevented from moving axially out of the female dovetails in the turbine rotor wheel by keys disposed in grooves in the outer circumference of the turbine rotor wheel and recesses in the sides of the buckets. However, due to the radial insertion of the closure bucket, it may not be possible to insert the keys the recesses of the closure bucket. Furthermore, while such keys can be used to secure most of he buckets around the circumference of the turbine rotor wheel, and the use of these keys further limits any axial movement of the buckets, they compound the radial insertion problem of the closure bucket.
The foregoing and other deficiencies of the conventional techniques are addressed by the system and method for securing an integral closure bucket to a turbine rotor wheel assembly of the present invention.
The present invention relates to a system and method for securing a radially inserted integral closure bucket to a turbine rotor wheel assembly. According to the assembly method the buckets, adjacent the closure bucket location on the turbine rotor wheel, can be spread apart from one another in both an axial and tangential direction to provide sufficient clearance for the radial insertion of the closure bucket. To provide the necessary clearance between the adjacent buckets in the tangential direction, a spreading fixture may be employed. The spreading fixture attaches to each of the adjacent buckets and can be adjusted to exert outward tangential force on each of the adjacent buckets so that the tangential clearance between the adjacent buckets increases enough to allow insertion of the closure bucket. Simultaneously, hydraulic jacks may be employed to move the adjacent buckets in opposing axial directions. One hydraulic jack moves a trailing bucket in a first axial direction while a second hydraulic jack moves a leading bucket in an opposite axial direction. Thus, the spreading fixture and hydraulic jack open sufficient tangential and axial clearance between the leading and trailing buckets to allow radial insertion of the closure bucket. Once the closure bucket is properly situated, the spreading fixture and hydraulic jacks can be removed.
In order to ensure that the leading and trailing buckets can be moved a sufficient distance to permit the insertion of the closure bucket, the conventional locking keys are not used. Instead, channels may be milled in the bottoms of the female dovetails in the turbine rotor wheel. Twist locks can be inserted in these channels. Prior to insertion of the closure bucket, the twist locks can be unlocked, so that the trailing and leading buckets can be spread apart. Once the closure bucket is properly situated, the twist locks may be rotated 180 degrees to prevent the leading and trailing buckets from moving axially. A twist lock is also used to secure the closure bucket. In order to provide enough play between the leading and trailing buckets to obtain sufficient clearance for the closure bucket, up to three leading and three trailing buckets may have twist locks provided in milled channels.